3471 


izx  ICtbrtH 


SEYMOUR  DURST 


When  you  leave,  please  leave  this  book 

Because  it  has  been  said 
"Sver'thing  comes  t'  him  who  waits 

Except  a  loaned  book." 


Avery  Architectural  and  Fine  Arts  Library 
Gift  of  Seymour  B.  Durst  Old  York  Library 


PLATE  I. 


F.  J.  H.  Merrill,  Photo. 

Precambrian  granite,  Breakneck  Mt.,  N.  Y. 
Photomicrograph  in  polarized  light,  enlargement  22  diameters. 


Digitized  by  the  Internet  Archive 
in  2014 


http://archive.org/details/geologyofcrystalOOunse 


PLATE  II. 


F.  J.  H.  Merrill,  Photo. 

Precambrian  granite,  King's  quarry  near  Garrisons,  N.  Y. 
Photomicrograph  in  polarized  light,  enlargement  22  diameters. 


PLATE  III. 


(V. 1 


[Reprinted  from  Arezu  York  state  museum  report,  i8qb~\ 

APPENDIX  A 


THE  GEOLOGY  OF  THE  CRYSTALLINE  ROCKS  OF 
SOUTHEASTERN  NEW  YORK. 

By  Frederick  J.  H.  Merrill. 

This  paper  contains  a  synopsis  of  the  results  of  an  investigation 
begun  in  1883  and  carried  on  at  personal  expense  till  1890,  from 
which  time  till  1893,  small  sums  were  afforded  from  the  funds  of  the 
New  York  State  Museum  for  continuing  the  field  work.  In  1895, 
there  being  no  museum  funds  available  for  this  purpose,  the  Director 
of  the  United  States  Geological  Survey  contributed  $200  for  com- 
pleting the  Precambrian  and  Palaeozoic  geology  of  the  Harlem 
sheet  of  the  United  States  topographic  atlas,  embracing  the  region 
about  New  York  city.  A  copy  of  this  Geologic  map  and  of  the 
descriptive  text  were  furnished  to  Director  Walcott  and  will  be 
incorporated  in  the  New  York  Folio  which  is  soon  to  be  published. 

The  preliminary  results  of  the  general  investigation  were  pub- 
lished in  the  American  Journal  of  Science,  series  3,  vol.  XXXIX, 
p.  389.  The  geologic  mapping  of  the  whole  area  east  of  the  Hudson 
in  New  York  was  published  in  the  Economic  and  Geologic  map  of 
New  York,  by  F.  J.  H.  Merrill,  and  in  the  Preliminary  Geological 
Map  of  New  York*,  compiled  and  published  by  W.  J.  McGee,  under 
the  direction  of  James  Hall,  State  Geologist. 

The  geology  of  Westchester  county  is  also  shown  on  a  scale  of 
four  miles  to  the  inch,  in  the  Geologic  Map  of  a  Part  of  Southeastern 
New  York,  by  F.  J.  H.  Merrill,  published  in  Bulletin  No.  15,  of  the 
New  York  State  Museum,  which  also  forms  part  of  the  48th  annual 
report  of  the  New  York  State  Museum.  This  bulletin  also  contains 
the  Economic  and  Geologic  Map. 

In  completing  the  work  for  the  Harlem  sheet  of  the  New  York- 
folio  the  writer  had  an  opportunity  to  review  the  ground  in  com- 

*The  draft  of  the  southeastern  crystalline  area  for  this  publication  was  prepared  at  the  request 
of  Prof.  Hall  in  1893  and  was  not  revised  before  engraving,  so  that  it  differs  slightly  from  the 
map  of  Southeastern  New  York  which  contains  the  results  of  additional  field  work. 


M38m-Jag8-2oo 


22 


NEW  YORK  STATE  MUSEUM 


pany  with  Prof.  C.  R.  Van  Ilisc  and  secure  his  criticism  and  ap- 
proval of  the  principal  points  discussed. 

With  the  permission  of  the  Director  of  the  United  States  Geo- 
logical Survey,  the  data  obtained  under  his  auspices  in  the  field  work 
of  completing  the  Harlem  sheet  are  herewith  incorporated. 

THE  CRYSTALLINE  ROCKS  OF  SOUTHEASTERN 
NEW  YORK. 

The  crystalline  rocks  of  southeastern  New  York  lie  on  the  east 
of  the  Hudson  River,  in  New  York,  Westchester,  Putnam  and 
Dutchess  counties,  from  whence  they  extend  into  Connecticut;  and 
on  the  west  of  the  river,  in  Orange  and  Rockland  counties,  whence 
they  extend  southwesterly  into  New  Jersey.  The  lowest  member  is 
a  coarse  hornblende  granite  which  forms  the  central  mass  of  the 
range  of  mountains  known  as  the  Highlands  of  the  Hudson,  and,  in 
their  highest  peak,  Breakneck  Mountain,  is  exposed  through  a  ver- 
tical height  of  nearly  1,200  feet.  (PI.  I.)  With  these  greater 
masses  of  hornblende  granite,  are  associated  other  local  masses  of 
granite  comparatively  free  from  hornblende,  which  are  extensively 
used  for  building  stone.  (PL  II.)  These  granites  are  probably 
igneous  and  of  great  age,  and  on  their  flanks  are  banded  gneisses 
consisting  chiefly  of  quartz  and  orthoclase  with  biotite  and  horn- 
blende, containing  numerous  beds  of  jna^iictic  iron-ore.  The 
gneisses  on  the  south  side  of  the  Highlands  (PI.  Ill)  extend 
through  Westchester  county  in  a  series  of  folds  with  south- 
westerly trend,  and  on  the  northern  slope  of  the  Highlands  at  several 
places  in  Dutchess  county,  are  overlain  unconformably  by  Palaeozoic 
basal  quartzites,  which  are  believed  to  be  of  Cambrian  age  and  are 
bordered  by  Ordovician  limestone  and  slate  or  schist.  Some  of  the 
principal  valleys  of  Putnam  county  contain  belts  of  limestone  asso- 
ciated with  quartzite  and  mica  schist,  which  are  probably  to  be  cor- 
related with  similar  rocks  hereafter  described  as  altered  palaeozoic 
strata. 

From  the  relation  of  the  quartzite,  limestone  and  schist  of  West- 
chester county  to  the  underlying  gneiss,  which  is  precisely  similar 
to  that  of  the  Palaeozoic  strata  in  southern  Dutchess  county  and 


REPORT  OF  THE  DIRECTOR  23 

Putnam  county  to  the  subjacent  gneiss,  and  from  the  nearly  complete 
stratigraphic  continuity,  it  is  inferred  that  the  crystalline  limestone 
of  Westchester  county  is  equivalent  to  that  of  southeastern 
Dutchess  county,  the  age  of  which  has  been  satisfactorily  established 
by  the  work  of  Dwight,  Dana,  and  others  to  be  Calciferous-Trenton, 
and  the  schist  and  micaceous  gneiss  overlying  the  limestone  by  like 
analogy  is  considered  to  be  of  Hudson  river  age. 

Besides  the  older  granites  just  mentioned,  there  are  in  West- 
chester and  New  York  counties  many  later  eruptive  rocks  of  con- 
siderable areal  importance. 

Prominent  among  them  is  a  red  granite  consisting  chiefly  of  quartz, 
orthoclase  and  biotite  which  is  injected  into  and  through  the  gneiss 
at  many  points,  and  at  Sing  Sing,  through  the  overlying  limestone. 
In  Yonkers  township  is  a  large  area  of  reddish  granite  quite  gneissoid 
in  texture,  which  is  intrusive  in  the  Fordham  gneiss. 

The  mica  schist  has  been  specially  subject  to  igneous  intrusions. 
Within  its  areas  occur  the  Cortland  series  of  diorites  and  norites 
described  by  J.  D.  Dana*  and  Geo.  H.  Williams, f  the  Harrison 
diorite  described  in  detail  by  H.  Ries,t  the  serpentines  which  are 
altered  eruptives  and  certain  gray  granites  which  occur  in  domes, 
bosses  and  lenses  in  the  southernmost  part  of  Westchester  county. 
Near  the  shores  of  Long  Island  Sound  the  Manhattan  schist  is 
everywhere  injected  with  bands,  lenses  and  dykes  of  pegmatite, 
granite,  amphibolite  and  pyroxenite. 

All  the  stratified  crystalline  rocks  above  described,  with  the  pos- 
sible exception  of  the  Fordham  gneiss,  were  originally  sediments  laid 
down  in  horizontal  strata,  the  quartzite  representing  a  beach  deposit, 
the  limestone,  a  deposit  in  water  unaffected  by  wash  from  the  land 
and  probably  of  warmer  temperature,  and  the  schist  a  deposit  of 
sandy  mud  in  shoaler  water.  These  three  rocks  form  a  reliable 
record  of  a  period  of  subsidence  of  the  land  and  transgression  of  the 
sea  with  subsequent  recession  and  emergence. 

*Am.  Jour.  Sci.  Ill,  Vol.  xxii  1881  pp.  1C3-11Q. 
t       "  "  "   xxxi  1886  pp.  26-41. 

xxxiii  1887  pp  135-144;  191-199. 
$  Trans.  N.  Y.  Acad.  Sci.,  Vol.  xiv.,  1895  pp.  80-86. 


24  NEW  YORK  STATE  MUSEUM 

At  a  time  or  at  times  not  accurately  determined,  but  which  prob- 
ably began  not  later  than  the  Upper  Silurian,  and  may  have  contin- 
ued at  intervals  to  the  end  of  the  Palaeozoic,  these  horizontal  strata 
by  lateral  pressure  were  thrown  into  parallel  folds  throughout  a 
broad  belt  of  country  having  a  general  northeasterly  trend  and  with 
the  Palaeozoic  beds,  the  underlying  rocks  of  greater  age  were  also 
folded.  As  the  cross  sections  show,  the  folds  are  closely  compressed 
and  in  many  cases  are  overthrown  to  the  eastward  and  westward,  so 
that  frequently  the  rocks  on  both  sides  of  the  fold  dip  in  the  same 
direction.  Associated  with  the  longitudinal  folding  of  these  rocks 
was  a  transverse  folding,  the  general  result  of  which  was  elevation  at 
the  northward,  so  that  the  parallel  ridges  with  their  intervening 
valleys  as  a  rule,  pitch  or  slope  very  gently  to  the  southwest.  There 
are  local  variations  from  this  general  condition  and  some  of  the  folds 
have  locally  a  northward  pitch,  but  the  general  condition  may  be 
noticed  in  the  western  ridges  of  Fordham  gneiss  which  in  the  town  of 
Yonkers  attain  a  height  of  300  feet,  and  on  Manhattan  Island  pass 
below  the  sea  level  and  do  not  reappear. 

As  already  stated  these  rocks  may  be  classified  in  the  following 
manner : 

CRYSTALLINE  ROCKS. 

Ordovician: 

1  Manhattan  schist,  containing  garnet  fibrolite,  kyanite  and 

staurolite. 

2  Inwood  limestone,  crystalline  dolomite,  containing  diopside 

and  tremolite. 
Cambrian : 

Lowerre  quartzite  1 

f  Algonkian?    Fordham  gneiss 
6  d  I 

«"§!,,  (  Gneisses 

£       Archaean  ] 

t  <  Granites 

For  detailed  examination  of  these  formations,  it  has  been  neces- 
sary to  depend  largely  on  the  exposures  along  the  east  shore  of  the 
Hudson  river  and  those  in  southern  Westchester  county  and  on 
New  York  Island.  In  central,  eastern  and  northern  Westchester 
county,  the  quaternary  deposits  of  stratified  and  unstratified  drift 


REPORT  OF  THE  DIRECTOR 


25 


are  so  thick  and  extensive  that  the  outcrops,  are  few.  The  locali- 
ties discussed  are  therefore  chiefly  confined  to  the  vicinity  of  New 
York  city. 

The  stratified  crystalline  rocks  within  the  area  under  consideration, 
which  is  south  of  the  41st  parallel,  belong  to  two  principal  divisions, 
the  Precambrian  and  the  Palaeozoic.  Of  the  Precambrian  only 
one  member  can  be  recognized,  which  has  been  called  the 
Fordham  gneiss.  Of  the  Palaeozoic  there  are  two  persistent  mem- 
bers, the  Inwood  limestone  and  Manhattan  schist,  and  a  third  of  local 
and  slight  development,  the  Lowcrre  quartzite,  which  underlies  the 
limestone. 

PRECAMBRIAN. 
Fordham  Gneiss. 

The  Fordham  gneiss,  named  from  the  former  town  of  that  name, 
within  which  it  is  well  exposed,  is  a  gray  banded  gneiss  varying  much 
in  the  composition  of  its  bands  or  layers,  which,  as  a  rule,  are  quite 
thin,  rarely  exceeding  two  inches  in  thickness.  Some  of  these  are 
highly  quartzose  (PI.  IV.),  some  are  largely  composed  of  biotite  and 
some  consist  of  pegmatite  or  granite  which  has  been  injected  parallel 
to  the  regular  banding  of  the  gneiss.  Hornblende  is  an  occasional 
constituent  of  this  rock  but,  though  highly  persistent  in  some  bands, 
does  not  occur  over  large  areas  of  country.  Garnet  is  present  rarely 
in  but  small  quantity. 

As  the  schistosity  of  the  Fordham  gneiss  has  usually  a  very  sleep 
dip  the  exposures  of  this  rock  chiefly  show  cross  sections  of  the 
banding. 

It  is  difficult  to  give  this  rock  formation  a  systematic  name  which 
exactly  indicates  its  age.  If  it  is  of  sedimentary  origin  it  may  be 
called  Algonkian,  but  it  can  only  certainly  be  said  that  it  is  Precam- 
brian. 

The  Fordham  gneiss  forms  the  high  anticlinal  ridge  which  borders 
the  New  York  shore  of  the  Hudson  River  from  Yonkers  southward 
to  Spuyten  Duyvil  and  also  that  on  the  west  side  of  the  Bronx  valley. 
The  former  ridge  terminates  on  the  south  at  Spuyten  Duyvil  and 
does  not  reappear  on  Manhattan  Island.    The  latter  is  bifurcated  at 


26 


NEW  YORK  STATE  MUSEUM 


the  southern  end  and  the  western  fork  interrupted  by  a  cross  fold  at 
the  Harlem  River,  ends  on  Manhattan  Island  in  the  low  ridge  which 
borders  Seventh  avenue  on  the  west  at  One  Hundred  and  Fifty-fifth 
street,  and  disappears  by  pitching  below  the  general  surface  level 
about  half  a  mile  southward.  The  eastern  fork  which,  owing  to  the 
same  cross  fold,  disappears  beneath  the  limestone  in  Morrisania, 
reappears  near  the  Bronx  Kills  in  Mott  Haven,  where  it  forms  a  low 
anticlinal  ridge  interrupted  by  the  Kills  and  represented  on  Manhat- 
tan Island  by  a  few  outcrops  below  high  water  mark  at  the  foot  of 
East  123rd  and  125th  streets  which  are  now  obliterated.  Some  nar- 
row anticlinal  ridges  of  Fordham  gneiss  are  seen  on  the  islands  in 
the  East  River,  notably  Blackwell's,  Ward's,  N.  Brother's  and  S. 
Brother's,  and  it  is  the  only  stratified  crystalline  rock  at  present 
exposed  on  Long  Island,  where  it  may  be  seen  near  the  court  house 
in  Long  Island  City  and  at  intervals  on  or  near  the  shore  of  the  East 
River  from  Ravenswood  to  Lawrence's  Point.  It  is  also  found  in 
deep  well  borings  and  is  the  subterrane  of  western  Long  Island. 

PALAEOZOIC. 

At  the  base  of  the  metamorphosed  Palaeozoic  limestone  and  over- 
lying the  Fordham  gneiss  is  a  stratum  of  thinly  bedded  quartzite. 
This  deposit  occurs  in  southern  Westchester  county  near  Lowerre 
station  in  Yonkers,  at  the  Hastings  marble  quarry  and  about  one- 
quarter  mile  south  of  Sparta  on  the  shore  of  the  Hudson  River.  It  is 
well  shown  north  of  Peekskill  along  the  east  shore  of  Annsville 
Cove  and  in  the  valley  of  Peekskill  Hollow  Creek  near  Oregon. 
It  does  not  exceed  sixteen  feet  in  thickness  at  Hastings.  From  the 
name  of  the  southern  locality  this  is  called  the  Lowerre  quartzite. 
Its  age  is  probably  Cambrian  and  possibly  Georgian. 

INWOOD  LIMESTONE. 

This  is  one  of  the  most  prominent  formations  of  the  region  mapped 
and  is  a  coarsely  crystalline  dolomite,  distinctly  bedded  and  contain- 
ing at  many  localities  the  lime-magnesia  silicates,  diopside  and  tremo- 
lite,  and  occasionally  tourmaline.  Of  its  maximum  thickness  little 
is  definitely  known.    At  Tuckahoe  a  thickness  of  one  hundred  and 


PLATE  IV. 


REPORT  OF  THE  DIRECTOR 


27 


fifty  feet  is  shown  in  section.  In  the  Harlem  River  a  thickness  of 
about  seven  hundred  feet  is  indicated. 

The  age  of  this  limestone  is  probably  Calciferous-Trenton.  In  the 
absence  of  fossils,  which  could  not  have  withstood  the  extreme  meta- 
morphism,  the  exact  age  is  indeterminate. 

The  crystalline  limestone,  though  frequently  well  exposed,  must 
often  be  traced  by  its  absence  as  well  as  its  presence.  Its  solubility 
in  water  containing  carbonic  acid  renders  it  an  easy  prey  to  the  ele- 
ments, and  its  position  is  almost  everywhere  emphasized  by  low 
ground  ana  usually  by  deep  valleys.  Throughout  all  '.he 
principal  valleys  small  outcrops  may  be  found,  though  usu- 
ally for  considerable  distances  it  is  buried  in  river  gravel 
and  alluvium.  Where  it  has  undergone  the  maximum  of 
leaching  the  granular  particles  of  limestone  have  disappeared  entirely 
and  in  its  stead  we  find  a  mass  of  aluminous  and  magnesian  material, 
whitish,  green  with  scales  of  prochlorite,  red  with  peroxide  of  iron, 
and  sometimes  black  with  separated  carbon.  In  these  conditions  it 
is  often  mistaken  for  clay  or  kaolin,  and  was  thus  reported  from  the 
railroad  cutting  at  Morrisania,  from  the  Blackwell's  Island  tunnel 
and  from  dredgings  in  the  East  River  on  the  Middle  Ground,  Shell 
Reef  and  at  the  mouth  of  Newtown  Creek.  The  same  material  was 
also  found  overlying  the  Fordham  Gneiss  in  a  deep  boring  on  Tall- 
man's  Island  near  College  Point.  On  the  uplands  the  presence  of 
limestone  is  evidenced  by  coarse  yellowish  white  sand,  consisting  of 
partially  dissolved  cleavage  fragments  of  the  dolomite.  This  may 
be  seen  on  the  plain  east  of  Inwood. 

To  the  presence  of  the  limestone  is  due  the  commercial  promi- 
nence of  New  York,  as  all  the  navigable  channels  about  the  city 
are  submerged  valleys  which  owe  their  origin  to  the  solution  of 
the  limestone  along  the  lines  of  its  outcrop  and  exposure.  With- 
out the  submergence  the  limestone  valleys  would  not  be  navigable 
channels  and  without  the  presence  of  limestone  there  would  have 
been  no  valleys  for  the  submergence  to  render  navigable. 

Long  Island  Sound  near  New  York  owes  its  existence  to  the  same 
cause,  as  it  is  the  locus  of  a  broad  exposure  of  limestone  uncovered 
by  the  removal  of  the  Manhattan  schist  east  of  the  Westchester 
shore. 


28 


NEW  YORK  STATE  MUSEUM 


MANHATTAN  SCHIST. 

This  formation  covers  a  larger  area  than  any  other  within  the 
limits  of  the  Harlem  sheet,  and  is  the  uppermost  of  the  crystalline 
groups.  The  rock  is  essentially  a  mixture  of  biotite  and  quartz,  fre- 
quently containing  enough  orthoclase  to  give  it  the  composition  of  a 
gneiss.  The  principal  accessory  is  garnet,  which  occurs  in  crystals 
varying  from  one-sixteenth  to  one-quarter  of  an  inch  in  diameter. 
Occasionally  much  larger  crystals  are  found.  Fibrolite,  kyanite  and 
staurolite  are  also  frequent  accessories.  The  Manhattan  schist  has 
a  marked  schistosity  which  is  frequently  nearly  parallel  to  the  bed- 
ding, though  not  always. 

The  aspect  of  this  formation  is  intimately  affected  by  numerous 
igneous  intrusions  and  injections  of  granitic  and  basic  material, 
which,  in  some  places,  are  so  numerous  as  to  predominate  over  the 
schist.  The  small  masses  are  for  the  most  part  parallel  to  the  schis- 
tosity, though  in  part,  oblique  to  it.  The  larger  areas  usually  have 
their  longer  diameters  parallel  to  the  strike  of  the  schistosity.  They 
are  most  abundant  near  the  shores  of  Long  Island  Sound. 

As  the  geologic  map  shows,  in  southern  Westchester 
county,  the  Manhattan  schist  is  the  prevailing  rock  east 
of  the  limestone  valley  in  which  lies  the  New  York  and 
Harlem  Railroad.  This  eastern  area  is  closely  folded  and  its  bedding 
planes  are  mostly  on  edge.  It  terminates  at  its  southern  extremity 
in  a  closely  pressed  synclinal  fold,  pitching  northward,  which  crosses 
Randall's  Island  and  Ward's  Island  and  ends  at  Little  Mill  Rock  in 
Hell  Gate.  Flood  Rock,  which  was  removed  in  the  improvement  of 
Hell  Gate  channel,  was  part  of  this  synclinal.  On  Mill  Rock  the 
schist  is  much  injected  with  amphibolite  and  pegmatite. 

The  Manhattan  schist  is  also  the  prevailing  rock  on  New  York 
Island. 

IGNEOUS  ROCKS. 

Under  this  head  are  classified  those  rocks  which  are  clearly  intru- 
sive in  the  Fordham  gneiss,  the  Inwood  limestone  and  the  Manhat- 
tan schist. 

So  far  as  we  know,  they  belong  to  one  general  period  of  igneous 
activity,  the  time  of  which  can  not  be  stated  with  greater  exactness 


REPORT  OF  THE  DIRECTOR 


29 


than  that  it  was  posterior  to  the  deposition  of  the  Manhattan  schists 
and  therefore  post-Hudson  River,  and  prior  to  at  least  a  part  of  the 
dynamic  disturbance  and  crumpling  of  these  rocks  with  which  the 
intrusives  have  become  schistose  and  even  crumpled.  The  igneous 
rocks  which  occur  in  the  Precambrian  and  Palaeozoic  within  the 
region  south  of  the  41st  parallel  of  latitude  may  be  classified  as 
follows : 

Yonkers  gneiss 

Granites,  red  and  grey 

Pegmatite  dykes,  very  coarse 

Harrison  diorite 

Amphibolites  and  pyroxenites 

Serpentines,  derived  from  basic  intrusives. 

YONKERS  GNEISS. 

In  an  article  on  the  Metamorphic  Strata  of  Southeastern  New 
York*,  the  writer  called  attention  to  a  reddish  gneiss  which  appeared 
to  be  the  lowest  stratum  in  that  terrane.  From  the  microscopic 
structure  of  this  rock,  studied  at  certain  localities,  and  from  its  appar- 
ent relations  to  the  overlying  gray  gneiss,  the  conclusion  was  formed 
at  that  time  that  it  was  a  metamorphosed  sedimentary  rock.  More 
extended  observations  on  this  formation  made  during  the  summer  of 
1891  showed  that  it  was  not  uniformly  persistent  as  a  basal  member  in 
southern  Westchester  county,  and  that  it  was  not  limited  to  the  axes 
of  the  eroded  anticlinals.  The  fact  that  it  was  overlain  by  a  varying 
thickness  of  the  gray  gneiss  was  noticed  by  the  writer  at  an  early  date 
but  was  attributed  to  unequal  repetition  of  the  gray  gneiss  by  folding. 
Later  investigations  showed  that  a  rock  of  the  same  composition 
occurred  frequently  as  an  intrusive  either  in  veins  and  dikes  or  in 
bosses  like  the  one  at  Sparta. 

The  Yonkers  gneiss  is  technically  a  gneissoid  granite.  (PI.  V.) 
It  is  a  well  foliated  rock  consisting  of  quartz,  reddish  orthoclase  and 
biotite  with  a  little  plagioclase.  It  is  plainly  intrusive  in  the  Ford- 
ham  gneiss  and  has  become  completely  schistose. 


♦Am.  Jour.  Sci.  Ill,  Vol.  XXXIX,  p.  389. 


3° 


NEW  YORK  STATE  MUSEUM 


In  the  particular  area  where  this  rock  has  its  greatest  extent  it  has 
been  subjected  to  greater  dynamic  action  than  elsewhere  and  has 
been  reduced  to  a  gneissoid  condition. 

The  persistence  of  reddish  orthoclase  in  this  rock  suggests  that 
it  has  sprung  from  a  common  source  with  the  numerous  dykes  of 
red  pegmatite  and  granite  of  similar  composition  which  penetrate 
the  schist  and  limestone  at  many  points  in  Westchester  county. 

GRANITES. 

Gray  and  reddish  granites  in  small  dykes  oblique  to  the  banding  of 
the  gneiss  and  schists  are  quite  abundant,  but  of  more  frequent  occur- 
rence are  lenses  and  injections  of  granite  and  pegmatite  parallel  to 
the  banding  of  the  schistosity.  Bosses  of  pegmatite  frequently  occur 
in  the  Manhattan  schist.  A  granite  area  of  considerable  size  occurs 
near  Union  Corners  and  many  have  been  found  on  New  York  Is- 
land, which  are  now  built  over  and  concealed  from  view.  The  small 
islands  and  reefs  in  the  upper  Bay  and  most  of  those  in  Long  Island 
Sound  owe  their  existence  to  intrusions  of  granite  and  other  erup- 
tives  in  the  schist,  which  by  their  hardness  have  resisted  erosion. 

PEGMATITE  DYKES  AND  BOSSES. 
These  are  intrusions  of  coarse  granitic  material  in  dykes  and 
bosses  from  one  to  ten  feet  in  diameter.    They  are  most  abundant 
in  the  Manhattan  schist. 

HARRISON  DIORITE.* 

This  rock  is  intrusive  in  the  Manhattan  schist  in  the  town  of  Har- 
rison and  consists  of  orthoclase,  plagioclase,  quartz  and  hornblende. 
A  smaller  area  of  similar  rock  occurs  at  Ravenswood,  L.  I.,  where 
it  outcrops  in  a  long  narrow  ridge  of  northeasterly  trend  and  is 
intrusive  in  the  Fordham  gneiss. 

The  mass  which  forms  Milton  Point  near  Rye  has  been  subjected 
to  much  dynamic  action  and  is  well  banded.  The  same  rock  is 
abundant  along  the  shore  of  Long  Island  Sound  between  Portchester 
and  Greenwich  and  is  abundant  in  Greenwich  and  Stamford  town- 
ships. 


*  H.  Ries Trans.  N.  Y.  Acad.  Sci.  1895  Vol.  xiv  pp  80-86. 


PLATE  V. 


F.  J.  H.  Merrill,  Photo. 

Sheared  granite  (Yonkers'  Gneiss),  Hastings,  N.  Y. 
Photomicrograph  in  polarized  light,  enlargement  22  diameters. 


REPORT  OF  THE  DIRECTOR 


31 


AMPHIBOLITES  AND  PYROXENITES. 
Intercalated  with  the  Manhattan  schist  and  also  with  the  beds  of 
the  Fordham  gneiss,  we  find  at  a  great  number  of  localities  on  New 
York  Island  and  in  Westchester  county,  hornblendic  and  augitic 
bands  and  lenses  of  limited  thickness,  usually  only  a  few  feet.  In 
composition,  these  rocks  resemble  diorites  and  diabases,  and  in 
structure  they  are  granular,  and  though  they  are  at  present  in  a 
foliated  condition,  their  general  characters  suggest  that  they  were 
originally  eruptive  rocks.  Locally  the  magnesian  silicates  in  these 
rocks  are  altered  into  epidote. 

SERPENTINES. 

A  large  number  of  observations  have  been  made  on  these  inter- 
esting rocks,  the  result  of  which  are  given  in  the  following  paper. 


APPENDIX  B 


THE  ORIGIN  OF  THE  SERPENTINES  IN  THE  VICINITY 
OF  NEW  YORK. 

By  Frederick  J.  H.  Merrill. 

This  paper  was  written  in  1890,  as  part  of  a  thesis  for  the  degree 
of  Doctor  of  Philosophy,  at  Columbia  College.  It  has  been  with- 
held from  publication  a  long  time,  in  the  hope  of  making  it  more 
complete,  but  an  opportunity  for  this  not  having  been  offered,  the 
paper  is  published  in  its  original  form,  leaving  to  future  time  the 
completion  of  the  investigation. 

LIST  OF  PAPERS  ON  SERPENTINE  CONSULTED  IN 
THE  PREPARATION  OF  THIS  ARTICLE.  ' 


Beck   Mineralogy  of  New  York,  p.  275. 

Bonney   Quart.  Jour.  Geol.  Soc,  Vol.  XXXIII,  p.  884-928. 

"    Geological  Magazine  1877,  p.  59-64. 

"    Geological  Magazine  1879,  p.  362-371. 

"    Geological  Magazine  1882,  p.  571. 

"    Geological  Magazine  1887,  p.  65-70. 

Britton   Annals  N.  Y.  Acad.  Sci.  Vol.  II,  p.  161-184. 

Coquand   Bull.  Soc.  Geol.  de  France,  (3),  VII,  p.  27-44. 

Dana,  J.  D   Am.  Jour.  Sci.  (III.),  VIII,  pp.  454-455. 

 Am.  Jour.  Sci.  (III.),  XX,  pp.  30-32. 

Daubree   Geologie  Experimentale,  p.  542. 

Delesse   Annales  des  Mines  (5),  XII,  p.  509. 

"   Annales  des  Mines  (5),  XIII,  pp.  393, 

Dieulefait  ....  Comptes  Rendus,  XCI,  p.  100. 

Diller   Bull.  U.  S.  Geol.  Surv.  No.  38. 

von  Drasche...  Tschermak's  Min.  Mitt.  1871,  pp.  1-13. 
Emmons   Geol.  of  New  York,  p.  67-72. 

"    American  Geology,  I,  p.  43. 

Gentth   Second  Geol.  Surv.  ot  Pa.,  Azoic  Rocks. 

Gratacap  Am.  Jour.  Sci.  (Ill)  XXXII,  pp.  374~378- 

"   Proced.  Nat.  Sci.  Ass.  Staten  Island.    May  14,  1887. 


REPORT  OF  THE  DIRECTOR 


33 


Guembel.  ......  Sitz.  Muench.  Akad.  1886  (1),  pp.  25-70. 

Hitchcock  ....  Geol.  of  Mass.  II,  p.  614-619. 

Hunt,  T.  S. . . .  Second  Geol.  Surv.  of  Pa.,  Azoic  Rocks. 

 Am.  Jour.  Sci.  (II),  XVI,  p.  217. 

«•         ....  Am.  Jour.  Sci.  (II),  XIX,  p.  417. 
"         ....  Am.  Jour.  Sci.  (II),  XXV,  p.  217-226. 
....  Am.  Jour.  Sci.  (II),  XXVI,  p.  234-240. 

"   Am.  Jour.  Sci.  (II),  XL.  p.  49. 

"   Am.  Jour.  Sci.  (Ill),  I,  p.  182-191. 

"        ....  Geol.  Surv.  of  Canada.    1863,  pp.  47 1-591,  635. 
"        ....  Chem.  and  Geol.  Essays,    pp.  25,  122-3,296-318, 
33^  342,  347- 

"    Geol.  Hist,  of  Serpentines,  Trans.  Royal   Soc.  of 

Canada,  Vol.  I,  Sec.  IV,  1883. 


Hussak  Tschermak.  Min.  u.  Pet.  Mitt.  1882,  V,  p.  6i-8r. 

Mather   Geol.  1st  Dist.  of  N.  Y.,  pp.  283-285,  461-462. 

Merrill,  G.  P. .  Proceed.  U.  S.  Nat.  Mus.,  Vol.  XI,  pp.  105-111. 
"  ..  Proceed.  U.  S.  Nat,  Mus.,  Vol.  XI,  pp.  105-111. 

Muller   Neu.  Jahrbuch,  1846,  p.  257-288. 

Rogers,  H.  D. .  Geol.  of  Pennsylvania,  Vol.  I. 

Rose,  G   Pogg.  Ann.,  185 1,  Vol.  LXXXII,  p.  511-530. 

Rosenbusch,  H .  Mik.  Phys.  der  Pet.  Wicht.  Miner.  2te.  Aufl.  pp.  557-559. 

Roth,  J   Allg.  u.  Chem.  Geol.,  Vol.  I,  pp.  11,  113,  134. 

Sandberger.  . . .  Neu.  Jahrbuch,  1866,  p.  385-400. 

"         ....  Neu.  Jahrbuch,  1867,  p.  17 1-177. 
Scheerer   Pogg-  Ann.,  Vol.  LXVIII;  p.  319. 

 Am.  Jour.  Sci.  (II),  V,  p.  381-389. 

 Am.  Jour.  Sci.  (II),  VI,  p.  189-206. 

 Am.  Jour.  Sci.  (II),  XVI,  p.  217. 

Stapff   Profil.  Geol.  du  St.  Gothard,  etc. 

Tschermak   Sitz.  Wien.  Akad.,  LVI,  iste,  Abtheil. 

"         ....  Lehrbuch  der  Mineralogie  2te,  Aufl. 

Vanuxem   Third  Ann.  Rept.  Geol.  Third  Dist.  N.  Y.,  1839,  pp. 

260-3. 

"    Final  Rept.  Geol.  Third  Dist.  N.  Y.,  pp.  108-1. 

Weigand  Tschermak's  Min.  Mitt.,  1875,  p.  183-206. 

Whitney   Geol.  of  Lake  Superior,  II,  2. 

Williams,  G.  H.  Am.  Jour.  Sci.  (Ill),  XXXIV,  p.  137-145. 

Bull.  U.  S.  Geol.  Surv.,  No.  28. 
Zirkel   Zeitsch.  Deutsch.  Geol.  Gesell,  Vol.  XIX,  p.  68. 


34 


NEW  YORK  STATE  MUSEUM 


THE  ORIGIN  OF  SERPENTINE. 
The  origin  of  serpentine  rocks  was  for  a  very  long  time  a  matter 
of  controversy  among  geologists.  They  have  been  variously 
regarded  as  primary  deposits,  as  plutonic  rocks,  as  alteration  pro- 
ducts of  rocks  rich  in  anhydrous  magnesian  silicates  and  as  the  result 
of  metasomatic  change  in  rocks  of  any  kind  whatsoever,  by  the  sub- 
stitution of  a  hydrous  magnesian  silicate  for  some  other  silicate  or 
carbonate. 

About  the  middle  of  the  present  century  Bischof  and  Gustav  Rose 
expressed  the  unqualified  opinion  that  all  serpentines  were  of  second- 
ary origin,  but  as  they  were  chemists  rather  than  geologists  and  did 
not  undertake  to  discuss  the  various  geognostic  problems  involved  in 
the  study  of  these  rocks,  they  were  unable  to  account  for  all  the 
phenomena  associated  with  them,  and  their  views,  with  which  those 
of  many  other  chemists  and  mineralogists  coincided,  were  not 
accepted  by  geologists  as  conclusive. 

Since  most  of  the  minerals  from  which  serpentine  was  held  to  be 
derived,  viz.:  Olivine,  enstatite,  hornblende,  augite,  diallage  and 
chondrodite,  were  believed  to  belong  pre-eminently  to  eruptive 
rocks,  it  was  considered  inevitable  that  if  the  serpentines  were  all  of 
secondary  origin  they  must  have  been  derived  from  igneous  rocks, 
and  as  this  hypothesis,  in  many  cases,  did  not  agree  with  the  appar- 
ent stratigraphic  conditions,  it  did  not  gain  general  acceptance.  For 
example,  many  serpentines  were  found  distinctly  stratified  and  inti- 
mately associated  with  stratified  crystalline  -rocks,  some  of  which 
were  limestones  and  obviously  not  of  igneous  origin.  Dr.  T.  Sterry 
Hunt,  who  has  discussed  the  origin  of  serpentines  at  great  length,* 
while  conceding  the  alteration  of  olivine  and  enstatite  into  serpentine 
in  some  cases,  considers  the  association  of  serpentine  with  these  two 
minerals  to  be  an  evidence  of  the  simultaneous  development  of 
hydrous  and  anhydrous  silicates  from  a  magnesian  sediment  of  chem- 
ical origin  such  as  he  ascribes  to  all  crystalline  stratified  rocks.  He 
also  expresses  doubt  as  to  the  igneous  origin  of  many  of  the  olivine 
rocks  from  which  serpentine  is  held  by  others  to  be  derived. 

*  Trans.  Royal  Soc.  Canada,  Vol.  I,  §  iv,  1883. 


REPORT'  OF  THE  DIRECTOR 


35 


In  this  expression  of  doubt  as  to  the  igneous  origin  of  many  mag- 
nesian  silicate  rocks,  Dr.  Hunt  voices  the  sentiment  of  most  field 
geologists  who  have  made  such  rock  masses  a  subject  of  study.  All 
the  common  anhydrous  silicates  of  magnesia  are  found  to  occur  as 
individual  rock  masses  or  as  constituents  of  them  in  the  stratified 
crystalline  terranes.  In  the  opinion  that  anhydrous  and  hydrous 
silicates  of  magnesia  are  simultaneously  developed  from  a  magne- 
sian  sediment  or  magma,  Dr.  Hunt  seems  to  stand  alone  and  unfor- 
tunately for  those  who  would  give  due  weight  to  this  hypothesis, 
does  not  advance  any  conclusive  arguments  in  its  behalf,  nor  does 
he  record  any  observed  facts  in  connection  with  serpentine,  which 
make  it  apparent  that  this  mineral  is  ever  developed  simultaneously 
with  olivine  or  any  other  anhydrous  magnesian  silicate.  Dr.  Hunt 
alludes  to  the  studies  of  Scheerer  on  the  association  of  olivine  and 
serpentine  at  Snarum,  in  Norway,  which  led  that  eminent  scientist 
to  assert  his  belief  that  the  two  silicates,  hydrous  and  anhydrous, 
were  formed  simultaneously,  because  the  alleged  pseudomorphs 
were,  in  many  cases,  enclosed  in  masses  of  chromite. 

According  to  the  writer's  understanding  of  Prof.  Scheerer's 
article,*  the  only  ground  for  rejecting  the  idea  of  pseudomorphism 
was  that  if  the  crystals  of  serpentine  were  produced  by  the  hydration 
of  pre-existing  olivine  an  increase  of  bulk  would  ensue  which  must 
rend  apart  by  expansion,  the  matrix  of  the  altered  crystal,  many  of 
the  pseudomorphs  being,  as  already  stated,  enclosed  in  masses  of 
chromite  which  were  unfractured  and  conformed  exactly  to  the  sur- 
face of  the  included  crystal,  which  had  the  characteristic  form  of 
olivine  though,  in  its  external  portion  at  least,  it  had  the  composition 
of  a  serpentine. 

If  this  was  Scheerer's  reason  for  rejecting  the  idea  of  the  pseudo- 
morphism of  the  olivine  into  serpentine,  his  objection  can  not  be 
accepted  until  it  be  proven  that  the  chromite  masses  enclosing  the 
serpentine  were  formed  before  the  latter.  As  chromite  masses  are 
almost  exclusively  confined  to  serpentine  rocks  and  in  them,  accord- 
ing to  Tschermak,  are  formed  by  the  segregation  of  the  oxides  of 
iron  and  chromium  which  are  set  free  in  the  decomposition  of  the 


♦Pogg.  Ann.  Vol.  LXVlII,p.  319. 


36 


NEW  YORK  STATE  MUSEUM 


anhydrous  silicates  of  magnesia  from  the  alteration  of  which  he  hold3 
all  serpentines  to  be  derived,  it  may  be  that  the  chromite  matrices  of 
the  Snarum  pseudomorphs  were  deposited  about  them  after  their 
formation  and  were  subjected  to  little  or  no  expansive  force,  since, 
after  the  crystals  were  thus  enclosed  and  protected  from  external 
agencies  the  process  of  serpentinization  would  cease. 

Tschermak,  in  his  memoir  on  the  formation  of  serpentine,*  quotes 
the  words  of  Gustav  Rose  and  Volger  in  their  description  of  the 
Snarum  crystals.    Rose  writes  as  follows: 

"  Of  two  crystals  the  surface  is  dark  leek-green,  soft  and  a  perfect 
serpentine. "  On  the  freshly  broken  surface,  however,  the  fact  is 
evident  that  the  serpentine  is  only  from  one-half  to  two  lines  (i-4mm.) 
thick  and  graduates  into  a  very  light,  yellowish-green  mass  which 
traverses  the  crystal  irregularly  and  encloses  white  areas  with  a  high 
lustre,  which  are  so  hard  that  they  can  not  be  scratched  with  a  knife. 
These  also  appear  to  have  cleavage  surfaces,  but  the  whole  mass  is 
penetrated  with  fine  fissures,  and  the  small  individual  parts  are  bril- 
liant on  almost  every  side  so  that  the  true  cleavage  cracks  can  not  be 
determined. 

"  Through  another  crystal  passes  a  cleft  about  as  thick  as  a  sheet 
of  paper,  which  is  filled  with  very  finely  fibrous,  highly  transparent, 
leek-green  chrysotile.  From  this  spread  out  to  right  and  left,  fis- 
sures filled  in  a  similar  manner,  which  are  nearly  perpendicular  to  the 
principal  crevice  and  which  turn  back  upon  themselves  in  ramifica- 
tions which  are  sometimes  very  small  and  sometimes  from  one  to 
one  and  one-half  lines  (2-3111111.)  in  thickness. 

"  Where  the  margins  of  these  turn  back  quickly  they  often  touch 
and  intersect  themselves  and  the  whole  mass  between  them,  even 
when  hard  and  brilliant,  is  colored  green;  where  they  meet  each 
other  at  greater  distances  the  included  mass  is  white,  of  greater 
toughness  than  in  the  first  crystal  and  of  subconchoidal  fracture. 

"  It  is  here  evident  that  the  whole  mass  of  the  crystal  was  pene- 
trated by  fissures  which  became  filled  with  serpentine  and  from  which 
the  decomposition  has  proceeded  in  all  directions." 


*Sitz.  Wien.  Akad.  LVI.,  ist  Abtheil. 


REPORT  OF  THE  DIRECTOR 


37 


Prof.  Volger  from  his  examination  of  this  material  arrived  at  a  sim- 
ilar conclusion. 

Since  the  development  of  microscopic  methods  of  research  a 
large  number  of  eminent  investigators  have  studied  the  structure  and 
optical  properties  of  serpentine,  and  all,  including  Tschermak,  Ros- 
enbusch,  Kalowsky,  Websky,  Wiik,  Des  Cloizeaux,  Von  Drasche 
and  Fischer,  are  unanimous  in  considering  it  an  alteration  product  of 
anhydrous  magnesian  silicates.  Prof.  Rosenbusch  in  his  late  work 
on  rock-making  minerals,*  epitomizes  the  latest  knowledge  of  the 
subject  as  follows: — 

"  Serpentine,  according  to  the  mineral  from  which  it  is  derived, 
has  a  fibrous  or  apparently  lamellar  structure.  The  apparent  lamellae 
may,  however,  only  represent  parallel  bundles  of  fibres.  The  ar- 
rangement of  the  fibres  is  quite  varied.  They  are  sometimes  parallel 
and  sometimes  confusedly  felted  and  the  optical  characters  of  the 
fibres  between  crossed  nicols  change  with  their  dimensions  and  their 
arrangement.  In  parallel  aggregates,  which  are  not  too  finely 
fibrous,  one  may  recognize  with  certainty  that  they  are  biaxial  with 
very  large  axial  angles  the  negative  bisectrix  of  which  is  perpendicu- 
lar to  the  axis  of  the  fibre  which  is  the  axis  of  least  elasticity.  These 
fibres  have  a  weak  refractive  power  (very  near  that  of  Canada  balsam) 
and  not  inconsiderable  double  refraction.  Chrysotile  exhibits  these 
properties  very  clearly.  In  the  finely  and  confusedly  fibrous  aggre- 
gates very  nearly  complete  compensation  occurs  so  that  these  often 
appear  to  be  isotropic. 

"  The  mineral  from  which  serpentine  is  most  frequently  derived  is 
olivine.  The  alteration  begins  from  the  surface  and  from  the  crevices 
and  leads  to  a  fibrous  structure  with  simultaneous  separation  of  the 
iron  content  in  the  form  of  Fe2C>3,  2Fe203,  +  3H2O  and  Fe304- 
The  new  structures  of  greenish  to  yellowish-green  color  are  perpen- 
dicular to  the  crystal  boundaries  and  the  cracks.  Since  the  alteration 
takes  place  from  all  the  cracks  (which  cross  each  other  confusedly) 
and  from  the  sides  simultaneously,  an  olivine  in  which  the  alteration 
into  serpentine  has  begun,  appears  to  have  a  reticulate  structure.  The 
serpentine  strings  form  a  network  of  which  the  meshes  inclose  olivine 


*  Mik.  Phys.  der  Pet-  Wicht.  Miner,    ate  Aufl.  pp.  557-559 


38 


NEW  YORK  STATE  MUSEUM 


still  unaltered.  As  the  process  goes  on,  new  fissures  are  cleft  in  con- 
sequence of  the  increase  of  volume  associated  with  the  alteration 
and  thus  render  possible  the  constant  increase  of  the  new  structures 
until  the  olivine  is  completely  transformed.  " 

"In  the  alteration  of  hornblende  and  actinolite  into  serpentine  the 
cleavage  planes  of  amphibole  and  their  oblique  separations  are  very 
clearly  brought  out  in  the  arrangement  of  the  serpentine  bundles. 
Between  crossed  nicols  the  lines  of  parallel  fibres  differentiate  them- 
selves in  lively  colors  from  the  dark  ground  of  the  confusedly  fibrous 
field,  now  running  parallel  to  one  another  now  intersecting  one 
another  at  an  angle  of  1240  to  1250,  or  forming  rhombic  figures  with 
other  angles  and  rectangles.  There  results  in  this  way  a  structure 
which  is  characteristic  in  the  highest  degree  and  which  Wiegand 
designated  as  lattice  or  window  structure.  " 

"  Other  serpentines,  which,  microscopically,  occasionally  show  a 
schistose  structure,  under  the  microscope  appear  to  consist  of  foliated 
masses  which  cross  each  other  at  right  angles  and  so  show  a  netted 
structure.  "  "  These  serpentines  appear  to  have  resulted  from  the 
alteration  of  monoclinic  pyroxenes.  " 

From  the  alteration  of  the  rhombic  pyroxenes  is  produced  both 
serpentine  and  bastite. 

According  to  Tschermak  the  alteration  of  olivine  into  serpentine 
may  be  expressed  as  follows: — 

Olivine  consists  of  Mg2Si04  and  Fe2Si04  combined  in  varying 
proportions. 

2  (Mg2Si04)  +  CO2  +  2H20=2H20,  3MgO,  2Si02  +  MgC03. 
5  (Fe2Si04)  +  O  +  6H20=2Fe304  +  2Fe203,  3H2O  +  5Si02. 
A  small  portion  of  the  MgO  in  the  serpentine  is  replaced  by  FeO. 

SERPENTINE  LOCALITIES  NEAR  NEW  YORK. 

The  serpentines  in  the  vicinity  of  New  York  City  are  of  two  gen- 
eral classes: 

A.  Serpentine  masses  of  large  area. 

B.  Local  developments  of  serpentine  in  crystalline  magnesian 
limestones. 


REPORT  OF  THE  DIRECTOR 


39 


Of  the  first  class  the  most  extensive  is  that  of  Staten  Island,  N.  Y., 
next  in  size  are  those  of  Rye  and  New  Rochelle  in  Westchester 
County,  N.  Y.,  and  that  of  Castle  Point,  Hoboken,  N.  J.  A  fifth  area 
is  to  be  found  in  New  York  City,  on  West  6oth  Street,  between  ioth 
and  nth  avenues.* 

Of  the  second  class  the  most  prominent  examples  are  at  Montville 
and  Mendham  in  New  Jersey  and  others  are  found  in  the  continua- 
tion of  the  eastern  belt  of  crystalline  limestone  through  northeastern 
New  Jersey  and  Orange  and  Putnam  Counties  in  New  York.  The 
most  extensive  of  these,  on  the  east  bank  of  the  Hudson  River  near 
West  Point,  was  described  by  Mather  under  the  local  name  of  "cot- 
ton rock."f  He  mentions  besides,  other  localities  in  Putnam  County, 
notably  Huestis'  Quarry,  4^  miles  northeast  of  Cold  Spring.  In 
Westchester  County  serpentine  occurs  in  small  masses  at  the  Snow- 
flake  Marble  Quarry  at  Pleasantville. 

A  third  class  might  be  constituted  of  the  serpentine  which  is 
found  in  some  of  the  iron  mines  of  Putnam  County.  The  serpentine 
pseudomorphs  of  the  Tilly  Foster  Mine  have  been  described  at  length 
by  Prof.  J.  D.  Dana.  (Am.  Jour.  Sci.  III.  viii.  pp.  454,  455.) 

THE  NEW  ROCHELLE  SERPENTINE. 

The  serpentine  locality  of  Davenport's  Neck  at  New  Rochelle  has 
long  been  known  to  geologists  and  mineralogists.  Its  position  and 
stratigraphical  relations  are  shown  by  the  accompanying  map. 
(PI.  VI.) 

Prof.  J.  D.  Dana  from  his  observations  on  this  serpentine  con- 
cludes that  it  is  associated  with  a  limestone  bed  which  is  not  now  visi- 
ble, having  been  removed  by  solution.  The  writer's  study  of  the  local- 
ity suggests  that  the  serpentine  is  derived  from  magnesian  silicate 
rocks  intruded  in  the  Manhattan  schist.  (Am.  Jour.  Sci.  III.  xxxix. 
P-  391.) 

The  only  exposures  now  visible  are  at  the  northeastern  and  south- 
western extremities  near  the  water's  edge,  and  the  outcrops  have 
been  so  long  exposed  to  the  weather  that  the  process  of  serpentiniza- 


*  This  locality  is  now  covered  with  buildings. 

t  Cotton  rock  has  been  covered  by  the  railroad  embankment. 


40 


NEW  YORK  STATE  MUSEUM 


tion  is  complete  and  the  serpentine  itself  is  disintegrating.  The 
northeastern  outcrops  are  the  most  extensive  and  afford  the  most 
information  concerning  the  origin  of  the  deposit.  The  northern- 
most outcrop  is  dark  green  in  color,  of  very  coarse  texture  and  has 
been  derived  very  largely  from  bronzite,  small  quantities  of  which 
still  remain.  Succeeding  this  to  the  south  is  a  massive  rock  con- 
sisting of  hornblende  and  garnet  covering  an  area  of  about  one  hun- 
dred and  fifty  square  feet;  followed  in  turn  by  more  of  the  coarse 
serpentine,  some  of  which  is  reddened  by  the  oxidation  of  its  iron 
content.  For  one  hundred  and  fifty  feet  or  more,  the  rock  is  not 
visible  and  then  succeeds  a  fine  grained,  light  green,  rather  porous 
serpentine  mass  which  forms  a  small  promontory  near  the  middle  of 
the  area.  This  is  overlain  with  a  semblance  of  stratification  which 
does  not  harmonise  with  that  of  the  neighboring  gneisses,  by  a  red- 
dish serpentine  rock  containing  actinolite  in  various  stages  of  altera- 
tion. In  this  portion  of  the  deposit  are  thick  veins  of  deweylite  and 
chalcedony  and  a  considerable  amount  of  crystalline  calcite.  In  the 
opinion  of  the  writer  the  calcite  is  a  by-product  of  the  serpentiniza- 
tion. 

Microscopic  study  of  the  New  Rochelle  serpentine  suggests  that 
it  is  chiefly  derived  from  amphibole  and  bronzite.  The  occurrence 
of  fibrous  amphibole  and  bronzite  or  enstatite  in  connection  with  the 
serpentine  of  this  locality  has  already  been  recorded  by  Prof.  Dana. 
In  thin  sections  examined  by  the  writer,  crystals  of  bronzite  may  be 
seen  in  which  serpentine  has  been  formed  along  the  transverse 
crevices.  In  these  bronzite  crystals  are  found  also  irregular  masses 
of  pleonast,  the  relations  of  which  to  the  surrounding  mass  suggest 
that  they  are  of  secondary  origin.   (PI.  VII.) 

Where  actinolite  has  been  the  source  of  the  serpentine  the  un- 
altered mineral  verges  through  a  zone  of  brown,  partly  decomposed 
material  into  the  finely  fibrous  serpentine  which  contains  a  very  large 
proportion  of  minute  crystals  of  magnetite.    (PI.  VIII.) 

The  change  from  actinolite  into  serpentine  does  not  seem  to  be  so 
direct  as  that  from  bronzite.  The  zone  of  discoloration  appears  to 
represent  an  intermediate  stage  in  which  the  excess  of  iron  is  re- 
moved from  chemical  combination  with  the  silica  and  set  free. 


PLATE  VII. 


F.  J.  H.  Merrill,  Photo. 

Alteration  of  bronzite  into  serpentine.  Davenport's  Neck.  New 
Rochelle,  N.  Y. 
Photomicrograph  in  polarized  light,  enlargement  22  diameters. 


PLATE  VIII. 


J.  H.  Merrill,  Photo. 


Alteration  of  actinolite  into  serpentine,  Davenport's  Neck,  New 

ROCHELL.E,  N.  Y. 

Photomicrograph  in  polarized  light,  enlargement  22  diameters. 


REPORT  OF  THE  DIRECTOR 


41 


With  regard  to  the  genesis  of  the  minerals  from  which  this  serpen- 
tine is  derived  but  little  can  be  predicted;  there  seem  to  be,  however, 
some  reasons  for  not  considering  them  of  sedimentary  origin.  While 
the  writer  does  not  question  the  formation  of  amphibole  and  bronzite 
from  sedimentary  deposits,  the  evidence  of  such  origin  in  this  case  is 
not  conclusive.  It  is  impossible  at  present  to  predicate  with  certainty 
the  geological  character  of  the  primitive  rock.  The  outcrops  are  so 
limited  in  extent  and  so  far  advanced  in  alteration  that  the  writer  has 
found  no  clue  to  guide  him  in  his  investigation  of  this  point.  It  is 
safe  to  say  however  that  the  primitive  rock  mass  was  different  from 
any  now  known  in  an  unaltered  condition  in  this  terrane.  A  frag- 
ment of  bronzite  rock  was  found  by  Mr.  J.  I.  Northrup  in  the  debris 
removed  from  one  of  the  shafts  of  the  new  Croton  Aqueduct  near 
Tarrytown  and  this  may  have  been  a  part  of  such  rock  mass  as  that 
which  gave  being  to  the  New  Rochelle  serpentine,  but  unfortunately 
nothing  can  be  ascertained  concerning  its  source. 

The  origin  of  the  deposits  from  which  this  serpentine  and  its  con- 
geners have  been  derived  remains  the  most  important  question  con- 
nected with  their  history,  and  unfortunately  we  can  only  reason  upon 
analogy  in  discussing  it.  In  all  probability  the  magnesian  silicate 
rocks  which  by  their  alteration  have  yielded  these  serpentines  were 
similar  in  their  origin  to  the  amphibolites  and  pyroxenites  which 
abound  in  Westchester  county.  In  modern  sedimentation  no  evidence 
has  been  recorded  of  an  alteration  of  conditions  which  would  yield 
in  small  quantity  a  deposit  having  the  composition  of  a  magnesian 
silicate  when  immediately  before  and  after  it  the  sediment  was  chiefly 
composed  of  silica  and  aluminous  silicates.  In  composition  the  am- 
phibolite  and  pyroxenite  beds  of  the  Manhattan  Group  bear  the  same 
relation  to  the  strata  which  enclose  them  as  the  intrusive  mass  of  the 
Palisades  bears  to  the  beds  of  sandstone  and  arkose  between  which  it 
is  now  included.  There  is  nothing  but  their  somewhat  foliated  con- 
dition to  suggest  that  they  are  of  sedimentary  origin  and  this  charac- 
teristic has  been  shown  to  result  frequently  from  dynamo-meta- 
morphism. 

The  former  hypothesis  that  serpentine  is  largely  derived  from  the 
alteration  of  magnesian  limestone  or  dolomite  does  not  seem  to  be 


42 


NEW  YORK  STATE  MUSEUM 


supported  by  recent  investigations.  In  the  literature  to  which  the 
writer  has  access,  the  only  recorded  instance  of  such  derivation  is 
that  of  a  pseudomorph  collected  from  the  Tilly  Foster  Mine,  the  form 
and  structure  of  which  suggested  to  Prof.  J.  D.  Dana  that  it  had  been 
derived  from  a  crystal  of  dolomite.*  Mr.  George  P.  Merrill  has 
happily  suggested,  in  the  case  of  the  Montville  serpentine,  that  the 
excess  of  silica  set  free  in  the  decomposition  of  the  diopside  has  to 
some  extent  combined  with  the  magnesia  of  the  enclosing  dolomite 
and  thus  formed  a  serpentine  in  addition  to  that  formed  by  the  altera- 
tion of  the  pyroxene.  It  is  evident  that  a  dolomitic  limestone  can 
only  yield  serpentine  through  the  action  of  silicated  waters,  while 
the  magnesia-iron  minerals  above  mentioned  will  yield  serpentine 
under  the  influence  of  ordinary  atmospheric  waters. 

In  the  present  opinion  of  the  writer  the  origin  of  the  New  Rochelle 
serpentine  has  been  as  follows: 

The  mica  schists  were  formed  by  sedimentation  and  meta- 
morphism.  The  amphibolites  and  other  magnesian  silicate 
rocks  were  intruded  and  by  subsequent  compression  at- 
tained their  foliated  structure.  Orographic  disturbance  subse- 
quently brought  the  strata  into  their  present  attitude  and  finally 
erosion  removed  the  covering  of  mica  schist  and  laid  bare  the  trun- 
cated folds.  Atmospheric  waters  then  had  free  access  to  the  mag- 
nesian silicates  and  the  process  of  de-ferrugination  and  hydration 
began  and  resulted  in  the  formation  of  the  serpentine.  The  excess  of 
silica  was  carried  off  and  deposited  in  the  form  of  chalcedony  of 
which  large  masses  and  minute  veins  occur  in  the  deposit,  and  the 
excess  of  iron  appears  as  magnetite  and  chromite,  while  according  to 
the  combinations  into  which  the  magnesia  entered,  various  varieties  of 
serpentine  were  formed,  together  with  magnesite,  talc  and  deweylite. 

The  writer  is  not  disposed  to  attribute  any  very  great  geological 
antiquity  to  the  serpentine.  Under  favorable  conditions  it  forms 
quite  rapidly.  At  Stony  Point  on  the  Hudson  River,  the  writer  has 
observed  a  surface  of  peridotite,  which  had  apparently  been  swept 
clean  by  the  ice  sheet,  covered  by  a  layer  of  serpentine  about  one-* 


•Since  the  above  was  written  some  material  has  been  given  to  the  writer  by  Dr.  Hunt  of  the 
Brooklyn  institute,  which  in  appearance  corroborates  Prof.  Dana's  conclusion. 


REPORT  OF  THE  DIRECTOR 


43 


fourth  of  an  inch  thick.  As  in  this  case  only  a  small  portion  of  the 
rock  had  by  its  decomposition  yielded  serpentine,  and  the  layer  ob- 
served by  the  writer  had  been  leached  out  of  the  rock,  it  is  reasonable 
to  infer  that  a  considerable  depth  of  the  rock  might,  since  the 
glacial  epoch,  have  been  changed  into  serpentine  had  all  of  its 
minerals  yielded  that  alteration  product. 

From  what  is  known  of  the  erosion  of  the  region  about  New  York 
it  may  be  inferred  that  the  rock  mass  which  yielded  the  New  Rochelle 
serpentine  was  exposed  to  the  action  of  atmospheric  agencies  not 
earlier  than  the  Mesozoic  age. 

THE  STATEN  ISLAND  SERPENTINE. 

The  general  characteristics  and  extent  of  this  deposit  have  been 
described  by  Dr.  N.  L.  Britton,  but  the  question  of  its  origin  is  :.ot 
entirely  settled.  Throughout  most  of  its  extent  this  area  has  suf- 
fered so  complete  an  alteration  as  to  yield  no  traces  of  the  mineral 
from  which  it  was  derived.  Mr.  Gratacap  records  the  presence  of 
traces  of  unaltered  hornblende  in  specimens  from  Bard  avenue  and 
elsewhere.  In  a  well  boring  made  through  the  serpentine  some 
fibrous  amphibole  or  actinolite  was  found  in  a  comparatively  unal- 
tered condition.  (Trans.  N.  Y.  Acad.  Sci.,  Vol.  I,  p.  58.)  Dr.  A. 
A.  Julien  (loc.  cit.)  states  that  he  has  found  traces  of  unaltered 
hornblende  in  Staten  Island  serpentine. 

In  none  of  the  material  which  the  writer  has  examined  has  he 
found  unaltered  particles  of  the  primary  mineral,  but  there  is  fre- 
quently present  a  reticulate  structure  similar  to  that  which  Roscnbusch 
has  described  and  illustrated  as  characteristic  of  serpentine  derived  from 
olivine.  In  a  large  number  of  sections  examined  by  the  writer  the 
"  lattice  structure  "  characteristic  of  serpentine  derived  from  horn- 
blende was  wanting,  the  angles  between  the  cracks  being  more 
nearly  those  of  pyroxene  than  of  hornblende.  It  is  not  improbable 
that  more  than  one  magnesian  silicate  has  contributed  to  its  origin. 

In  the  process  of  alteration  limonite  and  free  silica  were  the  chief 
by-products.  The  former  appears  in  the  once  extensive  bed  of 
limonite  which  has  been  used  as  an  iron  ore,  and  the  latter  in  the 
groups  of  quartz  crystals  which  so  frequently  occur  in  the  former. 


44 


NEW  YORK  STATE  MUSEUM 


If  there  were  any  considerable  amount  of  alumina  in  the  primitive 
minerals  it  was  probably  carried  off  and  deposited  with  the  limonite. 
The  manner  in  which  this  ore  originated  accounts  for  its  freedom 
from  sulphur  and  phosphorus. 

THE  SERPENTINE  OF  RYE  AND  HOBOKEN. 

After  studying  a  large  amount  of  material  from  these  two  localities 
the  writer  is  unable  to  contribute  any  new  facts  regarding  their 
origin.  The  outcrops  are  so  far  decomposed  as  to  afford  no  traces  of 
the  primitive  mineral.  Dr.  A.  A.  Julien  (loc.  cit.)  mentions  his  dis- 
covery of  traces  of  hornblende  in  serpentine  from  Hoboken. 

SERPENTINES  ASSOCIATED  WITH  LIMESTONES. 

The  distribution  of  these  serpentines  has  already  been  described. 
The  deposits  at  Montville,  N.  J.,  are  of  much  interest.  Here  the  ser- 
pentine is  derived  from  segregated  masses  of  diopside  enclosed  in  the 
magnesian  limestone. 

Mr.  G.  P.  Merrill's  monograph  on  the  subject*  shows  very  clearly 
the  chemical  relations  of  the  primitive  mineral  and  its  alteration 
product,  and  discusses  one  of  the  few  cases  of  the  formation  of  ser- 
pentines from  the  magnesia  of  a  dolomitic  limestone.  At  Mendham 
the  serpentine  is  of  similar  origin  to  that  of  Montville  and  pyroxenes 
easy  of  decomposition  occur  throughout  this  belt  of  limestone  of 
which  the  extent  has  already  been  mentioned. 

In  Westchester  County,  N.  Y.,  at  Pleasantville  and  elsewhere  the 
serpentine  is  likewise  derived  from  magnesian  silicates,  chiefly  of  the 
pyroxene  group  but  occasionally  having  the  characteristics  of  an 
olivine. 


•Proceed.  U.  S.  Nat.  Mus.,  Vol.  XI. 


